She is one of the main villains of the novel, obsessed with revenge against the Evrémondes. She ruthlessly seeks revenge against the Evrémondes, including Charles Darnay, his wife Lucie Manette and their child, for crimes a prior generation of the Evrémonde family had committed. These crimes include the deaths of her sister, brother, and father. She refuses to accept the reality that Charles Darnay's father changed his ways by intending to renounce his title to the lands to give them to the peasants who worked on them, and his son Charles renounces his title to the lands; however, Charles' arrogant and snobbish uncle becomes the Marquis St. Evrémonde. His arrogance causes the death of an innocent child which makes him hated, and helps legitimize Defarge's rage. Her consuming need for revenge against the innocent Darnay and his wife brings about her fatal doom by her own weapon at the hands of Miss Pross.

Defarge symbolizes several themes. She represents one aspect of the Fates. The Moirai (the Fates as represented in Greek mythology) used yarn to measure out the life of a man, and cut it to end it; Defarge knits, and her knitting secretly encodes the names of people to be killed. Defarge also symbolizes the nature of the Reign of Terror during the French Revolution in which radical Jacobins engaged in mass political persecution of all real or supposed enemies of the Revolution who were executed on grounds of sedition to the new republic through the guillotine, particularly targeting people with aristocratic heritage.